#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>

#define s_i(a) scanf("%d",(&a))
#define p_i(a) printf("%d",(a))
#define endl printf("\n")
#define p_l(a) printf("%ld",(a))
#define s_l(a) scanf("%ld",(&a))
#define s_ll(a) scanf("%lld",(&a))
#define p_ll(a) printf("%lld",(a))
#define forn(l,r) for(i=l;i<r;i++)
#define p_u(a) printf("%u",a)
#define s_u(a) scanf("%u",(&a))
#define s_f(a) printf("%f",(&a))
#define s_lf(a) printf("%lf",(&a))
#define s_llf(a) printf("%llf",(&a))
#define forr(r,l) for(i=r;i>=l;i--)
#define p_s(s) printf(s)
#define p_p printf(" ")
#define sqr(a) (a)*(a)

int main(void) {
    int n,m;
    s_i(n);s_i(m);
    int mas[m+1][n+1];
    int i;
    int mas1[(int)500*500+1];
    int mas2[(int)500*500+1];
    forn(0,m){
        int j;
        for(j=0;j<n;j++){
            mas[i][j]=0;
            mas1[i*m+j]=mas2[i*m+j]=-1;
        }
    }

    int ans=0;

    int k;s_i(k);
    forn(0,k){
        int x,y;s_i(x);s_i(y);
        x--;y--;
        mas[x][y]=1;
        mas1[i]=x;
        mas2[i]=y;
    }
    int count_=n*m-k;
    int counter=0;
    while(count_>0){
        int l=0;
        forn(counter,counter+k){
            if(mas1[i]>0&&!mas[mas1[i]-1][mas2[i]]){
                mas[mas1[i]-1][mas2[i]]=1;
                count_--;
                mas1[counter+k+l]=mas1[i]-1;
                mas2[counter+k+l]=mas2[i];
                l++;
            }

            if(mas1[i]<m-1&&!mas[mas1[i]+1][mas2[i]]){
                mas[mas1[i]+1][mas2[i]]=1;
                count_--;
                mas1[counter+k+l]=mas1[i]+1;
                mas2[counter+k+l]=mas2[i];
                l++;
            }

            if(mas2[i]>0&&!mas[mas1[i]][mas2[i]-1]) {
                mas[mas1[i]][mas2[i] - 1] = 1;
                count_--;
                mas1[counter + k + l] = mas1[i];
                mas2[counter + k + l] = mas2[i] - 1;
                l++;
            }

            if(mas2[i]<n-1&&!mas[mas1[i]][mas2[i] + 1]) {
                mas[mas1[i]][mas2[i] + 1] = 1;
                count_--;
                mas1[counter + k + l] = mas1[i];
                mas2[counter + k + l] = mas2[i] + 1;
                l++;
            }
        }
        counter+=k;
        k=l;
        ans++;
    }

    p_i(ans);

    return 0;
}